1. Field of the Invention
This invention relates to gas supported bearings, and more particularly to high precision, high speed gas supported bearings.
2. Description of the Prior Art
FIGS 1A and 1B depict a polygon mirror scanning system having a rotating polygon mirror 10 mechanically coupled to a rotating cylindrical shaft 12. The lower end of shaft 12 is rotatably coupled to the scanner housing 14 by ball bearing 16; the upper end of shaft 12 is coupled to housing 14 by ball bearing 18. Seals 20 minimize the circulation of liquid lubricant discharged from ball bearings 16 and 18 during high speed operations.
A permanent magnet 22 is rigidly coupled to shaft 12. When energized, motor field windings 24 interact with magnets 22 to rotate shaft 12 and polygon mirror 10.
Such prior art ball bearing supported motor driven loads respond to dimensional irregularities in the ball and race assemblies of the ball bearing and adverse interaction with the liquid lubricant can generate polygon mirror scanning errors of ten arc seconds or greater depending on the spacing between the two supporting bearing assemblies. Even when selected elements of the ball bearing scanning assembly are custom machined and custom fitted, scanning errors generally cannot be reduced below about five arc seconds. Lube redistribution can contribute to rotational period instability (velocity stability).
The unpreventable circulation of liquid lubricant discharged by the ball bearings enters the interior of housing 14, contaminates the reflective facets of polygon mirror 10, particularly along the leading edge of each facet, and thereby degrades the reflectivity of the mirror. Periodically, the individual facets of polygon mirror 10 must be cleaned to remove contaminating lubrication.
The prior art herringbone bearing assembly illustrated in FIG. 2 includes a cylindrical bore 26 and a shaft 28. Shaft 28 includes discrete herringbone patterns designated by reference numbers 30 and 32. Each herringbone pattern must be formed with the highest possible precision in the outer surface of shaft 38. As illustrated by the edge of the sectional view of shaft 28 as designated by reference number 34, approximately fifty percent of the shaft surface area within the herringbone pattern area is removed so that only approximately fifty percent of the remaining shaft surface can form a load supporting surface between the rotating shaft and the uninterrupted, cylindrical surface of the sleeve bore 26. This sharply limited load supporting surface area drastically reduces the load supporting forces or bearing stiffness generated between shaft 28 and sleeve 26. As a direct result, the closely spaced surfaces of shaft 28 and sleeve bore 26 do not lift off and become airborne until the grooves become pressurized. From 0 RPM to lift off velocity, these two surfaces operate as a contact bearing and mechanically rub against each other generating significant frictional forces and bearing surface wear.
The herringbone air bearing depicted in FIG. 2 relies upon the air pumping action generated by the interaction between the relatively rotating sleeve bore 26 and shaft 28. Such pumping forces generate a flow of pressurized air in the direction indicated by arrows 36 flowing upward through the bearing surface and are discharged from air discharge port 38. Once appropriate pressurization has been established by the rotating sleeve assembly, the sleeve bore 26 becomes airborne relative to the crowned top of the shaft 28. Until liftoff occurs, the top of shaft 28 rubs upon and can create surface wear at the interface between the top of shaft 28 and the base of air discharge port 38.
Another disadvantage of herringbone air bearings of the type depicted in FIG. 2 is that they must be operated in a vertical orientation. Deviation from the desired vertical alignment on the order of ten degrees of inclination can create rapid bearing surface wear and can result in failure of the herringbone bearing assembly.
The high level of mechanical precision required to create the herringbone pattern in the surface of bearing shaft 28 contributes to a high manufacturing cost.